Field of the Invention
This invention relates to heat switches and more particularly to the sub-class of heat switches that are passively activated based on a temperature stimulus and switch rapidly between thermally conductive and thermally insulating states.
Description of the Related Art
As illustrated in FIGS. 1a and 1b, a heat switch 10 is a device that switches between a thermally conductive state 12 and a thermally insulating state 14 to provide thermal management for electronics and other temperatures sensitive devices between two surfaces 16 and 18. The conductance ratio between the thermally conductive and thermally insulating states being typically at least 10:1, and more preferably at least 50:1, but the need is design specific and lower ratios may still provide useful thermal performance improvements. As illustrated in FIG. 2, a sub-class of heat switches includes those that are passively activated based on a temperature stimulus (e.g., crossing a temperature threshold 20), and exhibit the ability to rapidly switch between the thermally conductive state 12 and thermally insulating state 14 (or vice versa). The term rapid refers to heat switching on the order of seconds to minutes, not minutes to hours, typically less than 1 minute, and most preferably less than 20 seconds when exposed to an extreme temperature environment.
Shape memory heat switches are based on shape memory materials that undergo a solid-state phase change from martensitic to austenitic crystal structure at a prescribed temperature that commonly yields growth or shrinkage of the material by approximately 3-6%. U.S. Pat. No. 7,752,866 uses a shape memory spring to make and/or break thermal contact between two surfaces via the linear spring extension and contraction, with all movement along the same line of motion as the spring action. Similarly, U.S. Pat. No. 6,404,636 uses a shape memory Belleville washer to translate a heat generating device in and out of contact with a heatsink. Here, the entire assembly housing the devices is moveable along the same line of motion as the Belleville washer. The usefulness of this approach is limited in that (1) a conductive thermal path through a solid exists between the hot and cold sides in both the thermally conductive and thermally insulating states [no pure air gap], diminishing the heat switching effect and (2) the mass of the housing to be translated is large in comparison to the spring and much of the spring energy will be required to translate the massive housing against opposing frictional forces (e.g. tracks, alignment features, etc.), diminishing the spring energy available to generate contact pressure at a thermal interface.
Gas-gap heat switches operate by maintaining a small gap between two components (<=1-mil). In the thermally conductive state, heat transfer between components occurs via gas-gap conduction and radiation. Evacuation of the gas between the two components using a temperature activated sorbent material switches the device to a thermally insulating state, limiting the heat transfer mode to pure radiation. An example of a gas-gap heat switch is disclosed in U.S. Pat. No. 4,771,823. Gas-gap devices provide a passive, temperature activated, heat switching means, but require extremely tight tolerances and up to an hour to passively switch between states.
Differential thermal expansion devices leverage the differences in the coefficient of thermal expansion of two different materials to make and/or break thermal contact between components at a prescribed activation temperature. This is commonly achieved using bimetallic strips that exhibit a deflection with change in temperature. U.S. Pat. Nos. 3,177,933 and 4,304,294 both utilize bimetallic strips to achieve a heat switching mechanism. The simple fact that common materials deflect by millionths of an inch per degree temperature change require these devices to either be of a very large size (and thus slow responding) or be exposed to extreme temperature differences.
A wedge-based mechanical locking mechanism or “wedgelock” 30 is illustrated in FIGS. 3a (locked) and 3b (unlocked). Wedgelock 30 comprises a shaft 32, a plurality of wedge segments mounted on the shaft with one wedge segment 34 pinned via pin 35 to the end of the shaft and the remaining wedge segments 36 configured such that they can move with respect to each other, and a fastener 38 that threads into the shaft from the non-pinned end and seats on a fixed shoulder 40 of the final wedge segment. Applying torque to fastener 38 moves (contracts) the plurality of wedge sections in the axial direction along the shaft, forcing at least one wedge segment to move (expand) radially (e.g., perpendicular to the axial motion) to provide a mechanical locking force between two surfaces 42, 44 perpendicular to the axis of the shaft. The mechanical locking force between the two adjacent surfaces 42, 44 is achieved using axial contraction of the shaft 32 and fastener 38 to redistribute the axial load into a radial load between the surfaces of the wedge segments 34, 36 and the adjacent surfaces 42, 44. Thus, the conventional wedge-based mechanical locking mechanism serves as fixed mechanical interface between two adjacent surfaces. Existing wedge-based mechanical locking mechanisms emphasize a single and consistent form-factor that is specific to standardized tray-mounted electronics.
U.S. Patent Pub. No. 2007/0253169 entitled “Wedgelock Device for Increased Thermal Conductivity of a Printed Wiring Assembly” includes at least one wedge segment that is configured to move at an acute angle with respect to another wedge segment to secure a printed wiring board in the slot of a heat sink chassis. The wedgelock device provides improved thermal performance by creating additional thermal paths from the printed wiring board to the heat sink chassis. This is accomplished by forming the top and bottom surfaces of the wedge segments with a trapezoidal shape instead of the conventional rectangular shape in which the wedge segment moves perpendicular to the other wedge segments and axis of the shaft.